The M1-A1, like most main battle tanks in service, are fully tracked vehicles. This means that instead of wheels with rubber tires the tank has up to 12 small un-powered metal wheels and one or two drive wheels on each side. A track of heavy metal links is looped over the un-powered wheels. Shaped like big gears, the drive wheels have thick metal spokes that fit in holes in each of the metal links. The metal links themselves have an extremely thick rubber pad that is affixed to the outside facing portion of the link.
The advantage of the tracked system over wheels lies in the surface area it provides the vehicle in contact with the ground. In a typical wheeled system, only a very small portion of surface area is in contact with the ground. An average four wheeled vehicle with 10 inch wide tires will only have approximately 160 square inches of surface contact. That figure will of course increase and decrease depending on the size of the tire, the amount of air in the tire and the vehicle's speed/lift ratio.
A tracked vehicle on the other hand, has a near continuous strip of surface contact from the front of the vehicle to the rear. An M1-A1 with a track width of 24 inches and length of about 25 feet has a surface contact area of approximately 14,400 square inches!
As you can well imagine this provides for a lot of traction. The 1500 horsepower gas turbine engine can propel the 69.54 ton M1-A1 to 20 mph in 7.2 seconds. On pavement the enormous amount of traction the track system provides can stop the vehicle from it's top speed of 42 miles per hour in less than 50 feet. That's an amazing two body lengths. As an aside, the only time I witnessed this sort of emergency braking the crew was injured. The commander was propelled forward so swiftly that his jaw was broken as he literally swallowed the recoil buffer of the turret mounted M-2 machine gun.
The track system also allows for a few other performance benefits that a wheeled system cannot offer. The multiple wheels allow for a multiple point suspension that allows the tank to cross many obstacles without suffering undue turbulence. The M1-A1 can cross rough terrain at a sustained speed of 30 mph. The track system also allows the M1-A1 to cross a 9 foot open ditch, and scale an impressive 42 inches vertically to 60 degrees of inclination.
Impressive, but the M1-A1 also has its drawbacks. The extremely high heat produced by the engine makes the tank very difficult to hide from thermal optic night vision. The engine seals have also been designed for high operating temperatures. The result is that when the engine is stopped or cold, the M1-A1 will often leak oil like a fountain. The vehicle's enormous mass and poor driver visibility also make it a trick to navigate in confined areas.
On a training mission I saw a driver attempt to back up his M1-A1 without the service of a ground guide. due to the massive weight of the vehicle he didn't notice when he struck the M-113 APC to his rear. The M-113 weighs in at a relatively svelte 11 to 15 tons, depending on configuration. By the time the M1-A1 driver stopped to find out what everyone was so frantically yelling about, the APC to his rear had been pushed back 20 feet and was inclined about 50 degrees onto one side.
U.S. Army. Publications 2000 http://www.army-technology.com/projects/abrams/specs.html